The present disclosure relates generally to membrane structures. More particularly, the disclosure relates to membrane structures having high flux and high selectivity. The disclosure also relates to methods of manufacturing membrane structures.
Porous membrane structures are extensively used in filtration, separation, catalysis, detection, and sensor applications. Membrane structures with extremely fine pore sizes may be required for filtration and separation applications. The selectivity of a membrane is typically determined by the membrane pore size. Highly selective membranes typically have small pores. However for a given thickness, the smaller the pore size, the smaller the membrane flux. Therefore, membrane layers with fine pores must be made thin to ensure sufficient permeate flux. Typically thin membranes with small pores are stacked on thicker substrates with coarser pores, which provide mechanical support but do not significantly increase the resistance to flow. In such membrane structures it is extremely difficult to get a defect-free interface between layers to ensure sufficient connectivity through the membrane structure. In spite of much effort, there is still a demand for membrane structures with fine pores exhibiting high permeance and high selectivity, along with methods to produce such membrane structures, especially on an industrial scale.
Inorganic membranes have been employed for a variety of gas separation applications, including hydrogen purification and CO2 sequestration. Inorganic membranes possess good thermal stability, chemical resistance, and a high compressive strength, all of which are favorable for use in harsh operating conditions. In addition, dense and microporous membranes (pore size, Øp<2 nm) may have very high or even close to perfect gas selectivities. Inorganic membranes are usually prepared as a top layer on multilayer structures to provide the membrane with sufficient strength while keeping the separative top layer as thin as possible to reduce flow resistance. The selective membrane layers are generally prepared to a thickness of 1 to 10 μm, but may be less than 50 nanometers thick.
A major limiting factor to the application of supported inorganic membranes, however, is the frequent occurrence of defects which limits reproducibility, stability, and the separation performance of the membranes. Many thin microporous and dense gas separation membranes studied in the past have likely been affected by defects, leading to a large variation in the reported membrane performance, underestimation of selectivity, and overestimation of permeance. While microporous gas separation membranes have Øp˜0.5 nm, defects in the membranes may have a size from ˜4 nm to 1 μm. Such meso- (2-50 nm) and macro-defects (>50 nm) have a deleterious effect on the gas separation performance due to significant contributions of Knudsen flow, viscous flow, or both to the overall flow. These mechanisms result in a low selectivity or no selectivity at all. Permeance as a result of defects is much higher than that of a defect-free membrane area, and may dominate overall membrane permeance, artificially increasing permeance measurements. Since gas transport in membrane supports is generally in the Knudsen regime, the overall selectivity of a supported membrane with defects easily approaches Knudsen values. Leaks through defects may also reduce the driving force for microporous transport by a spreading of the feed composition in a resistive support over a large area just below the membrane. Support spreading effects lead to a reduction in the chemical potential gradient over an area much larger than the actual defect area.
Two approaches to producing inorganic membranes are gas phase synthesis and colloidal deposition. A common gas phase synthesis technique, chemical vapor deposition (CVD), makes use of gas phase precursors which undergo oxidation or thermal decomposition in a heated reaction chamber to deposit membrane layers. Colloidal synthesis usually starts with the preparation of a stabilized dispersion or colloid which is then coated onto membrane supports and heat-treated. CVD can be conducted in a self-repairing mode but produces relatively dense, low permeance membrane structures. In addition, CVD methods require significant investment in equipment, and are not easily incorporated in a viable, continuous fabrication process. Colloidal deposition may be conducted quickly under ambient conditions, compatible with continuous fabrication methods known to those of skill in the art. Thin, high permeance structures may be readily made via colloidal deposition methods known to those of skill in the art, however, colloidal methods are not self-repairing and often result in a substantial number of membrane defects.
Defects may arise as a result of imperfections in the deposition method, the deposition surface, or both. Membrane support surfaces which contain sufficiently large defects, may result in the formation of large, pinhole defects after deposition due to inadequate bridging or coverage of the latent structural defects. Defects of this type are more likely to occur for thin membrane layers.
In addition to the high susceptibility of gas phase and colloidal synthesis methods to support defects, both methods often also suffer from environmental contamination during membrane fabrication processes. Colloidal synthesis methods are also typically affected by defects from bubble generation during dispersion preparation, colloidal instability, and the removal of additives. In addition, the removal of templating agents in ceramic membranes, especially zeolites, during calcination may also lead to the formation of intercrystalline defects such as grain boundary defects and cracks.
Post-synthesis treatment of defective membranes has been used as a method to reduce the overall effect of defects on permeance, selectivity, and other associated properties. For example, palladium (Pd) nanoparticle impregnation of microporous silica membranes has been found to improve the H2/N2 selectivity at high temperatures. However, the observed selectivity increase was attributed mostly to the affinity of Pd to H2. In related studies, the He/N2 and CO2/N2 selectivities showed only minor improvements (10-20%). Atomic layer deposition (ALD) has been used post-synthesis on mesoporous silica membranes to remove large defects and to improve gas separation performance. Counter-diffusion chemical liquid deposition (CLD) for defect patching and CVD post-synthesis modification have both been used with limited success to reduce defects in zeolites membranes. More recently, rapid thermal processing (RTP) has been applied to zeolite membranes to reduce grain boundary defects from template agent burnout. Cyclodextrin was found to improve the CO2/CH4 selectivity when utilized to fill intercrystalline defects as compared to the selectivities achieved with untreated SAPO-34 membranes.
Another possible post-synthesis method for the reduction of membrane defects may be the coating of defective membranes with a permeable polymer. In such a method, the polymer coating must have a high permeance with respect to the selective membrane layer, but a much lower permeance than any of the meso/macro defects present in the pre-treated membrane. Such a method may lead to the reduction of defect flow contributions and support spreading effects without significantly adding to the overall flow resistance. Permeable polymer layers may be able to restore membrane performance to near intrinsic values, which may have numerous practical applications.